Mike, that is what I would use.  I shoot .409 bullets in my 40-70 SS and I really don't see a need for anything larger. When I order a custom mould, I request for the cast bullet to be at bore diameter to .001" over.  The bullet will bump up upon ignition.  
Anything larger, could cause problems.  If when seated in the cartridge, there may not be enough room to release the bullet if your chamber neck is tight or it may be hard to chamber. If you have to size the bullet down, then you risk bending the nose and not having a concentric bullet. JMO Bob